1. Field of the Invention
The invention relates generally to the field of electric motors. More particularly, the invention relates to electric motors which employ commutators and brushes. Even more particularly, the invention relates to techniques for controlling and maintaining the speed of electric motors that employ commutators and brushes.
2. Discussion of the Related Art
Electric motors have been the major source of power for many different types of equipment since their early days. The power source can be either AC or DC. AC and DC performance differ in that the DC power source has both higher efficiency and output.
An electromagnet is the basis of an electric motor. The armature in the motor is an electromagnet made by coiling thin wire around two or more poles of a metal core. The armature has an axle, and the commutator is attached to the axle. The commutator is typically a pair of plates, which provide the two connections for the coil of the electromagnet, attached to the axle.
The ongoing reversing of the electric field by the motor is what makes it run and is accomplished by two parts: the commutator and the brushes. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are usually just two pieces of metal or carbon that make contact with the contacts of the commutator.
In applications using electric motors, it is often necessary to control the speed of the motors in the equipment in which they are installed. The speed may be required to be fairly constant, regardless of the load amounts placed upon the motor. In industrial applications, extremely accurate measurement of the speed is sometimes a necessity.
One conventional method of motor speed control is to use open loop control. In this method, the power input to the motor is controlled, but no feedback is allowed from the motor itself. The problem with this technology is that it causes the motor input power to stay constant, regardless of the load on the motor. Therefore, for cases of varying motor loads, it is possible for the motor speed to vary with varying motor loads. The controller has no way of detecting speed variations.
Another conventional method incorporates a feedback loop into the control mechanism. This approach usually uses a mechanical sensor, such as an optical disk or magnet, coupled to an optical or magnetic sensor, to measure the rotational speed of the motor. The problem with this approach is that the sensors are expensive and it requires a detailed installation of the sensors and related equipment. This results in a complicated machine to install and use.
There have been various designs that tried to overcome the limitations of the above mentioned methods. However, these have been limited in their implementation. Some are only suitable for motors featured in small household appliances, such as U.S. Pat. No. 4,673,860, which is incorporated herein by reference, while others are only useable for universal motors powered by AC currents, such as U.S. Pat. No. 5,986,417, which is incorporated herein by reference, and yet others are used with induction motors, which do not possess brushes.
Heretofore, the requirements of a simple motor speed control apparatus that is easy to install and use, which keeps the speed constant even under varying loads for an electric motor powered by an AC or DC power supply, and which utilizes a commutator and brushes have not been fully met. What is needed is a solution that simultaneously addresses all of these requirements.
Embodiments of the present disclosure address several, if not all, of the requirements mentioned above. The following embodiments are representative. Of course, the invention is not limited to these embodiments.
According to an aspect of the invention, a method comprises: improving the performance of an electric motor that employs a commutator and brushes including: sending power to the motor, reading a set of data from the motor using an analog to digital converter and/or digital I/O port and a microcontroller configured to execute digital signal processing software, setting a speed for the motor, and maintaining the speed for the motor based on the set of data as analyzed by the microcontroller. The motor armature current waveform contains spikes that occur every time the brushes pass over a gap in the commutator. The digital signal processing separates these commutator spikes from the other noise present in the current waveform. By digitally measuring the frequency or time between these spikes, an accurate indication of the motor""s speed is obtained. By sensing the time of spike occurrence and predicting the occurrence time of the next spike, the invention can utilize a solid state switch to momentarily interrupt the current to the motor, there by eliminating or reducing arcing between the commutator and the brushes. According to another aspect of the invention, an apparatus comprises: a current sensing device coupled in series to the motor to monitor the current of the motor, a solid state switch coupled to the microcontroller which controls the amount of current delivered to the motor, a high pass filter coupled to the current sensing device that removes the 60 Hz frequency component from the voltage of the current sensing device, an analog to digital converter and/or digital I/O port that sends digital output to the microcontroller, and a microcontroller to perform digital signal processing coupled to the solid state switch.
These, and other, embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. As will be recognized by those having ordinary skill in this art, many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements.